Identification of two distinct NADH oxidases corresponding to H₂O₂-forming oxidase and H₂O-forming oxidase induced in Streptococcus mutans

Two distinct NADH oxidases, corresponding to H₂O₂-forming and H₂O-forming enzymes were purified to homogeneity from Streptococcus mutans and their basic properties determined. The H₂O₂-forming enzyme was a tetramer with a subunit molecular mass of about 56 kDa and required flavin adenine dinucleotide (FAD) for full activity. The enzyme had an isoelectric point of 6.6 and exhibited optimal activity at pH 6.0. The H₂O-forming enzyme was a monomer with a molecular mass of 50 kDa and activity independent of exogenously added flavin. The enzyme had an isoelectric point of 4.8 and exhibited optimal activity between pH 7.0 and 7.5. Both enzymes oxidized NADH (K(m) 0.05 and 0.025 mM for the H₂O₂- and H₂O-forming enzyme, respectively) but not NADPH and contained 1 mol of FAD per monomer. Spectra of the oxidized enzymes exhibited maxima at 271, 383 and 449 nm for the H₂O₂-forming enzyme and 271, 375 and 447 nm for the H₂O-forming enzyme. Antibodies raised against the H₂O₂-forming enzyme or the H₂O-forming enzyme reacted with their corresponding antigen, but did not cross-react. The amino-terminal regions of the two enzymes had completely different amino acid sequences.